1. Field of the Invention
The present invention relates to an image forming apparatus, and, more particularly, to a color image forming apparatus, which includes a plurality of photosensitive members, upon each of which is formed a visible image, whereby a visible color image can be formed by overlapping visible images of different colors formed on the respective photosensitive members.
2. Description of the Related Art
In general, an image forming apparatus is an apparatus to develop a black-and-white image or color image on a printing medium, such as paper, according to an image signal. Examples of the image forming apparatus include laser printers, inkjet printers, copiers, facsimiles, and devices performing various combination of functions thereof. Representative image forming methods of these various image forming apparatuses include an electro-photographic method, in which an electrostatic latent image is formed on a photosensitive member via irradiation of light beam, and in which the electrostatic latent image is developed with developer and transferred onto a printing medium, and an inkjet method, in which liquid-phase ink is ejected onto a printing medium surface according to an image signal.
In an electro-photographic image forming apparatus, after charging a photosensitive member surface with a predetermined electric potential, a light beam is irradiated onto the photosensitive member, to form an electrostatic latent image by potential difference between those portions that have been irradiated and those that have not. A developer is applied to the electrostatic latent image, to form a visible image. After the visible image, formed on the photosensitive member, is transferred to a printing medium, heat and pressure are applied to the printing medium, to fix the visible image, of, e.g., powder developer, on the surface of the printing medium. In such manner, an image is recorded on a printing medium.
Broadly speaking, an electro-photographic image forming apparatus could be a monochromatic image forming apparatus to print a black-and-white image and/or a color image forming apparatus capable of printing a color image.
A color image forming apparatus is classified into a so-called multi-path type color image forming apparatus having a single photosensitive member, and a so-called single-path type color image forming apparatus having a plurality of photosensitive members corresponding to the number of developing units of different colors.
In a multi-path type color image forming apparatus, only one developing unit is operated during one revolution of the single photosensitive member, forming a single color visible image on the photosensitive member. The visible image formed on the photosensitive member is transferred before another developing unit of different color is operated. Thus, a multi-path type color image forming apparatus may be equipped with a single photosensitive member and a single light scanning device.
In a single-path type color image forming apparatus, on the other hand, different colors of visible images are sequentially formed, sometimes with a slight time difference, on the respective ones of the photosensitive members corresponding to each of the developing units. A light scanning device in a single-path type color image forming apparatus may be formed with a plurality of light passage windows corresponding to the respective photosensitive members.
When the light passage window(s) becomes contaminated with debris, for example, toner or dust particles or dirt, etc., the image quality may be adversely affected. The likelihood of such contamination increases with the increased number of light passage windows, e.g., in the case of a single-path type color image forming apparatus.
An attempt to prevent toner or dirt from a developing unit from contaminating the optical parts of a light scanning device is disclosed by Japanese Patent Laid-Open Publication No. 2000-0263839 to Susumu (“Susumu”). Susumu discloses an image forming apparatus that includes a plurality of laser beam passage windows each facing a photosensitive member, a shutter to block the passage window, a shutter mover, and a laser beam passage formed on the shutter so as to be located on an optical path of a laser beam within a movement range of the shutter, the laser beam passage located so as to be not below a developing point of the photosensitive member. The shutter is divided into a shutter for black, and a shutter for chromatic colors, which are operated independently.
However, the image forming apparatus of Susumu requires a separate drive motor to drive the shutter, and suffers from increased manufacturing costs and/or size of the image forming apparatus in order to provide a space for the drive motor.
Moreover, in the image forming apparatus disclosed by Susumu the shutter moves along a single common plane, and if the plurality of light passage windows are not likewise arranged on a single plane, there may be gaps between some of the plurality of light passage windows and the shutter. These gaps may lessen the effectiveness of the shutters in preventing contaminations of the passage windows, and are wasted dead spaces contributing to a poor space utilization.